The term “molecular electronics” has been used to describe phenomena or devices that include a molecule as a circuit element (1,2). The motivation for the field is the prospect of making extremely small (potentially one molecule) electronic components with a much wider range of functions than conventional semiconductor electronic devices. If molecular devices become practical, a wide variety of applications in microelectronics, computing, imaging and display technology and chemical sensing can be envisioned. The large majority of proposed molecular electronic devices are based on the Gold-thiol system (Au/thiol), in which organic mercaptans “self assemble” on a flat gold surface to form an ordered monomolecular layer (3–6). In many cases, scanning tunneling microscopy of the Au/thiol layer or of gold particles thereon reveal the electron transfer characteristics of the monolayer molecule. However, the Au/thiol system forms films with many pinhole defects, so only a very small region (less than 30×30 nm, typically) can be examined without pinholes that result in short circuits. An alternative approach involves placing a single layer of molecules between two metal or metal oxide surfaces using Langmuir-Blodgett technology (7,8). In these experiments, the current/voltage behavior of a layer of molecules may be obtained, with the current path extending through the molecule itself.
While these experiments demonstrate certain characteristics of molecules as electronic components, they have severe disadvantages when considered for practical uses. First, the apparatus required is extremely complex and difficult to use, and so far has only been successfully implemented on a limited scale in very sophisticated laboratories. Second, both approaches result in films with unavoidable defects that limit both the size and lifetime of the devices. Third, the Au/thiol or Langmuir-Blodgett approaches to binding the molecule to two conductors generate large energy barriers that reduce current flow. The sulfur atom represents an “aliphatic” barrier that decreases electronic coupling between conductor and molecule. The Langmuir-Blodgett approach requires metal oxide films that purposely decouple the molecule from the conductor in terms of electronic interactions. When the molecule is electronically decoupled from the conductor, many potentially valuable applications of molecular electronics are prevented. Fourth, STM interrogates one or a few molecules at a time, so the massive parallelism inherent in microelectronic devices is difficult to conceive. These fundamental problems inherent in Au/thiol or Langmuir Blodgett devices prevent any conceivable practical application in the foreseeable future.
Independent of the field of molecular electronics was the development of methods for covalent bonding of molecular monolayers to carbon substrates such as carbon fibers and polished glassy carbon (9–11). These methods led to a robust monolayer that is conjugated with the carbon substrate through a strong carbon-carbon bond. Unfortunately, applications to molecular electronics are not possible with known technology because the surfaces are too rough. No one has succeeded in making a contact to the top of the monolayer because of substrate roughness that is much greater than the thickness of the monolayer. Only recently (12) has anyone made a carbon surface which is both smooth on a molecular scale, and amenable to covalent bonding of molecular layers.
None of the prior art meets the requirements for a practical molecular electronic device. They all are too difficult to make, prone to defects and pinholes, unstable, and require exceedingly sophisticated laboratory equipment to fabricate and study. In order to make a practical molecular electronic device based on molecular monolayers, the following requirements (at least) must be met:    1. The monolayer should be sufficiently flat and pinhole-free, so as to reduce or prevent short circuits.    2. There should be covalent bonding between at least one (and preferably both) of the conductors and the monolayer, so as to increase electronic coupling.    3. The monolayer may be a conjugated organic molecule, which in turn is conjugated with the pi electron system in the conductive substrate, making the resulting electronic coupling quite different from that in Au/Thiol layers.    4. The chemical bond of the monolayer to the conductive substrate should be strong and stable, and preferably not subject to oxidation in air.    5. After a metal layer is deposited to the top of the monolayer (by chemical deposition, vapor deposition, or electrodeposition) the junction is no longer an electrochemical system, and does not require ion motion or a solution.    6. It should be possible to encapsulate, possibly after fabrication of a complex circuit pattern, so as to make possible the fabrication of microelectronic junctions and devices using them.